1. Field of the Invention
The present invention relates to a power supply apparatus and heating apparatus and image forming apparatus More specifically, the present invention relates to a system for controlling supplied power using a zero-crossing signal as a trigger, and particularly to a heating apparatus for fusing a toner image generated by electrophotographic process on a recording medium, and to an image forming apparatus having the heating apparatus.
2. Description of the Related Art
Conventionally, an image forming apparatus using electrophotographic process has been known. The image forming apparatus fuses an unfixed image (toner image) formed on transfer paper through image forming means such as electrophotographic process on the transfer paper with a heat fuser. As heat fusers, such heat fusers as described in References 1-16 are known: they include a heat roller type heat fuser using a halogen heater as its heat source, or a film heating type heat fuser using a ceramic face heater as its heat source. As for the References, they will be enumerated below.
Generally, power is fed to these heaters from an AC power supply via switching devices such as triacs.
In the fuser using the halogen heater as its heat source, the temperature of the fuser is detected by a temperature detecting device such as a thermistor. In response to the temperature detected, a sequence controller carries out the ON/OFF control of the switching devices, that is, the ON/OFF control of supplying power to the halogen heater, thereby performing the temperature control in such a manner that the temperature of the fuser becomes a target temperature.
On the other hand, the fuser using the ceramic face heater as its heat source utilizes the temperature difference between the temperature detected by the temperature detecting device and a preset target temperature. More specifically, a sequence controller applies the temperature difference between the temperature detected by the temperature detecting device and the preset target temperature to a PI or PID control formula, thereby calculating a manipulated variable. The manipulated variable is a power ratio to be supplied to the ceramic face heater. From the power ratio calculated, the corresponding phase angle or wave number is determined. Then, according to the phase or wave number determined, the switching devices undergo the ON/OFF control, thereby controlling the temperature of the fuser.
To achieve the phase control, it is necessary to inform the sequence controller of the zero-crossing signal which is the trigger signal of the phase control as proposed by the References 17-21, for example. The zero-crossing signal refers to a pulse signal that has a rising or falling edge occurring at the timing corresponding to the zero-crossing point at which the AC input power supply changes its polarity. In other words, the zero-crossing signal is generated by detecting the zero-crossing points at which the AC input power supply changes its polarity from positive to negative or vice versa. Alternatively, the zero-crossing signal is generated by detecting that the voltage of the AC input power supply falls below a certain threshold voltage including the zero-crossing point. The zero-crossing signal is delivered to the sequence controller. After the time period corresponding to a determined phase angle has elapsed from the edge of the zero-crossing signal which is the pulse signal, the sequence controller turns on and off the switching devices, thereby carrying out the ON/OFF control at the specified phase angle.
The heat roller fusing type heat fuser has as its basic structure a heat roller (fusing roller) serving as a heating rotator and an elastic press roller serving as a pressing rotator pressed to the heat roller. The heat roller fusing type heat fuser rotates the pair of rollers so that a recording medium serving as heated material on which an unfixed image (toner image) is formed and supported is introduced into and passed through the pressing nip portion (fusing nip portion) between the pair of the rollers. As the recording medium, a sheet, dielectric-coated paper, electrofax paper and printing paper are known. In this way, the heat roller fusing type heat fuser fixes by heat and pressure the unfixed image on the recording medium by the heat from the surface of the heat roller and the pressure of the pressing nip portion as a permanently fused image.
As for the film heat type heat fusers (on-demand fusers), they are proposed by the References 1, 2, 3 and 12. These on-demand fusers bring a heat resistant film (fusing film), which is a rotator for heating, into intimate contact with a heater element with a pressing rotator (elastic roller) to transport the film with sliding. Subsequently, the on-demand fuser introduces the recording medium that carries the unfixed image into the pressing nip portion formed by the heater element and the pressing rotator with sandwiching the heat resistant fusing film, thereby transporting the unfixed image together with the heat resistant film. Then, the on-demand fuser fuses the unfixed image on the recording medium as a permanent image by the heat from the heater element fed via the heat resistant film and the pressure at the pressing nip portion.
The film heat type heating apparatus can use a low heat capacity linear heater element as its heater element, and can use a low heat capacity thin film as its film. Thus, the film heat type heating apparatus can reduce power consumption and wait time (achieve quick start). In addition, as the film driving method of the film heat type heating apparatus, the following methods have been known: a method of providing a driving roller inside the film; and a method of driving the film using the frictional force between the film and the pressing rotator used as the driving roller. Recently, the pressing rotator driving system has been used more often because it can reduce the number of components and cost.